A parallel read-out type high speed image sensor that simultaneously reads out charge signals from a plurality of read-out lines is utilized in order to capture high speed images. However, an in-situ storage image sensor is suitable for further increasing image capturing speed. The in-situ storage image sensor serially overwrites charge signals to image signal storages provided in the periphery of each pixel for recording during image capturing without reading-out the charge signals. The in-situ storage image sensor records charge signals in parallel simultaneously in all of the pixels as analog signals, thereby achieving a great increase in image capturing speed.
The present inventor has previously proposed an in-situ storage image sensor (slanted CCD-type image sensor) provided with charge signal storages made of linear charge coupled devices slantly extending from each of photodiodes having comparatively great areas (see Japanese unexamined patent publication 2000-165750).
FIG. 11 shows the principle of this slanted CCD-type image sensor. In FIG. 11, photodiodes are denoted as 1, CCD charge storages respectively provided with a plurality of elements 2a are denoted as 2, and drain gates are denoted as 4. Charge signals generated in each photodiode 1 are stored in elements 2a of the corresponding CCD charge accumulator 2 according to the order of generation (image capture order), as shown by numerals 1 to 5 attached to elements 2a in FIG. 11.
The slant of the CCD charge storage 2 with respect to the center axis line L1 of photodiodes 1 is significant feature.
If the CCD charge storages 2 extended parallel to the center axis line L2 of photodiodes 1, the CCD charge storage 2 extending from one photodiode 1 would need to be shifted to the right in the figure by the width of one CCD charge storage 2 for preventing interference with the photodiode 1 located directly below this photodiode 1 in the figure. As a result, in the case of FIG. 12, the row direction and the columnar direction of photodiodes 1 are not perpendicular to each other so that the arrangement of photodiodes 1 is distorted. In contrast to this, in the case of FIG. 11, since CCD charge storage 2 are slanted with respect to the center axis line L1 as described above, photodiodes 1 can be arranged so that both rows and columns have constant intervals and the row direction (direction of X axis) and the columnar direction (direction of Y axis) are perpendicular to each other. In other words, photodiodes 1 can be arranged in a right-angled grid pattern.
FIG. 13 shows one example of the above described slanted CCD-type image sensor. Each CCD charge accumulator 2 extends from the upper edge to the lower edge of the photo-receptive area in a gradually meandering manner and passes through regions 8 in gaps between two photodiodes 1 adjacent to each other in the columnar direction. Further, each CCD charge accumulator 2 is divided into segments corresponding to the number of regions 8 between photodiodes 1 that each CCD charge accumulator 2 passes through. Each of segments has input gate 3 at its upper end and a drain gate 4 at its lower end. Furthermore, the lowest edge of each CCD charge storages 2 is connected to a horizontal read-out CCD 6 provided outside of the photo-receptive area.
At the time of image capturing, the charge signals produced in each photodiode 1 are transferred by the corresponding CCD charge storage 2 and discharged out of the sensor from the drain gate 4. Further, at the time of reading-out, the input gate 3 and the drain gate 4 are closed so that charge signals in each CCD charge storage 2 are transferred to the horizontal read-out CCD 6. After that, the charge signals are read out from the sensor by the horizontal read-out CCD 6 through an amplifier 7.
Then, driving voltages for transferring charge signal through a CCD charge transfer path will be described.
FIGS. 14 to 18 respectively show typical patterns of the driving voltages. As shown in FIGS. 14A to 18A, electrodes 11a, 11b, 11c and 11d usually made of polysilicon are provided on the photo-receptive area, and driving voltages are supplied to these electrodes 11a to 11d via metal wires 12a, 12b, 12c, and 12d provided on the photo-receptive area. FIGS. 14B to 18B show the relationship between the position on the CCD charge transfer paths 10 in the direction of extension and the potential, and FIGS. 14C to 18C show the relationship between time and variation in the driving voltage.
FIG. 14 shows the case of a driving voltage having three levels and three phases, and FIG. 15 shows the case of a driving voltage having two levels and three phases. In these cases, three types of electrodes 11a to 11c respectively corresponding to phase φ1, φ2, and φ3 are needed. In the case of FIG. 15, a voltage variation of 6 steps is needed as shown in Steps S0 to S6 in order to transfer a charge signal from one element 10a to the next element 10a. FIG. 16 shows the case of a driving voltage having two levels and four phases wherein four types of electrodes 11a to 11d corresponding to phase φ1, φ2, φ3, and φ4 are needed. In these cases, of FIGS. 14 to 16, it is not necessary to change the impurity doping profile in CCD charge transfer path 10 in the direction of charge transfer and CCD charge transfer paths 10 constituted only by N regions are provided in a substrate of a P region. According to these systems of FIGS. 14 to 16, the amount of charge that can be transferred is great so that a great dynamic range can be secured, but they are not suitable for high speed transfer.
On the other hand, FIG. 17 shows the case of a driving voltage having two levels and two phases, while FIG. 18 shows the case of a driving voltage having two levels and one phase. In these cases, portions of having low levels of impurity dopants and portions having high levels of impurity dopants are formed in alternation in the surface of CCD charge transfer path 10, therefor previously creating unevenness of the potential gradient in the direction of transfer of charge signals. Thus, when a driving voltage is applied to electrodes 11a and 11b, a potential profile in a step form is formed so that the charges are transferred to the downstream side due to inclinations in this potential profile. According to these systems of FIGS. 17 and 18, the amount of charges that can be transferred is small, but they are suitable for high speed transfer.
It is necessary to generate many charge signals in small pixels within a photo-receptive area of an image sensor and, therefore, it is preferable for the CCD charge storage to be able to transfer a great number of charges. On the other hand, high speed is required in the horizontal read-out CCD located outside of the photo-receptive area. In addition, since the horizontal read-out CCD is located outside of the photo-receptive area, the amount of transferred charges can be increased by increasing the width due to the existence of extra space.
Accordingly, systems of FIGS. 17 and 18 are ordinarily adopted for the CCD charge storage 2 within a photo-receptive area, while systems of FIGS. 14 to 16 are ordinarily adopted for the horizontal read-out CCD 6.
The same number of types of metal wires for supplying a driving voltage to CCD charge transfer paths are necessary as the number of phases of the driving voltage. Further, since the same type of metal wires need to be arranged in the same layer, cross arrangement of different types of metal wires requires arrangement of the different types of metal wires in two different layers isolated from each other. Furthermore, in the case of an image sensor, it is necessary to place metal wires for supplying a control voltage to an input gate and a drain gate.
FIGS. 19A to 19E show examples wherein electrodes 11a to 11c and metal wires 12a to 12c for supplying driving voltages to CCD charge storages of the image sensor are placed in the same metal layer. FIG. 19A shows the case wherein a driving voltage has one phase, FIG. 19B shows the case wherein a driving voltage has two phases, and FIG. 19C shows the case wherein a driving voltage has three phases. Further, FIG. 19D shows the case wherein the driving voltage has three phases and one type of metal wire 13a for supplying a control voltage is placed in the same layer as metal wires 11a to 11c. Furthermore, FIG. 19E shows the case wherein a driving voltage has three phases and two types of metal wires 13a and 13b for supplying a control voltage are placed in the same layer as metal wires 11a to 11c. In these figures, contact points are denoted as 17.